The present invention relates to a system and process for secondary oil recovery and, more particularly, to a system and process for secondary oil recovery in which the sub-surface boundary or interface between the to-be-recovered oil and the reservoir drive fluid is detected and controlled to optimize recovery, and, still more particularly, to a system and process in which anomalies within the gravitation field caused by density changes and contrasts consequent to the movement over time of the sub-surface boundary between the to-be-recovered oil and the reservoir drive-out or re-pressurizing fluid is monitored.
Oil and natural gas hydrocarbon reservoirs form as a consequence of the transformation of organic matter into various types of hydrocarbon materials, including coals, tars, oils, and natural gas. It is believed that oil and gas reservoirs form as lighter hydrocarbon molecules percolate toward the surface of the earth until they are trapped in a relatively permeable layer beneath a relatively impermeable layer that xe2x80x98capsxe2x80x99 the permeable layer. The lighter hydrocarbon molecules continue accumulating, often accompanied by water molecules, into relatively large sub-surface reservoirs. Since the reservoirs exist at various depths within the earth, they are often under substantial geostatic pressure.
Hydrocarbon resources have been extracted from surface and sub-surface deposits by the mining of solid resources (coal and tars) and by pumping or otherwise removing natural gas and liquid oil from naturally occurring sub-surface deposits.
In the last century, natural gas and oil have been extracted by drilling a borehole into the sub-surface reservoirs. In general, most reservoirs were naturally pressurized by the presence of free natural gas that accumulated above the liquid oil layer and, often, by water that accumulated below the liquid oil layer. Since naturally occurring crude oil has a density lower than that of water (i.e., ranging from 0.7 in the case of xe2x80x98lightxe2x80x99 crude oil to 0.9 in the case of xe2x80x98heavyxe2x80x99 crude oil), crude oil accumulates above the water-permeated layer and below the gas-permeated layer. Thus, a borehole terminating within the oil-permeated layer would yield oil that receives its driveout energy from an overlying gas-permeated layer and/or an underlying water-permeated layer.
In general, the xe2x80x98primaryxe2x80x99 recovery of crude oil occurs during that period of time that the natural pressurization of a reservoir causes the crude oil to be driven upwardly through the well bore. At some point in the operating life of the reservoir, the naturally occurring pressurization is effectively depleted. Several different methods, known generally as secondary recovery methods, have been developed to extract crude oil after natural pressurization is exhausted. In general, secondary recovery involves re-pressurizing the reservoir with a fluid (i.e., a liquid or a gas) to lower the oil viscosity and/or drive the remaining crude oil in the oil-permeated layer to the surface through one or more wells. The drive fluid is introduced into the reservoir by injection wells which pump the pressurized drive fluid into the reservoir to displace and thereby drive the oil toward and to the producing wells.
Various schemes have been developed for the placement of the injections wells. For example, a line of injection wells can be placed at or adjacent to a known boundary of the reservoir to drive crude oil toward and to the producing wells. As the boundary between the pressurizing fluid advances past the producing wells, those producing wells can be capped or, if desired, converted to injection wells. In another arrangement, injection wells are interspersed between production wells to drive the oil in the oil-permeated layer away from the injection point toward and to immediately adjacent producing wells.
Various fluids, including water at various temperatures, steam, carbon dioxide, and nitrogen, have been used to effect the re-pressurization of the reservoir and the displacement of the desired crude oil from its rock or sand matrix toward the production wells.
In the waterflood technique, water at ambient temperature is injected into a reservoir to drive the oil toward and to the producing wells. The injected water accumulates beneath the crude oil and, in effect, floats the lighter density crude oil upwardly toward and to the borehole of the producing well. In those cases where the oil-permeated layer is relatively thin from a geological perspective and is also confined between two relatively less permeable layers (i.e., an impermeable reservoir ceiling and a more permeable reservoir basement), water is injected at a relatively high pressure and volume to effect an xe2x80x98edge drivexe2x80x99 by which the crude oil is pushed toward the oil producing wells. Sometimes, the injected water is heated to assist in lowering the viscosity of the oil and thereby assist in displacing the crude oil from the pores of the permeable sand or rock. The waterflood technique is also well-suited for driving natural gas entrapped within the pores of relatively low-permeability rock to a producing well.
In the steamflood technique, steam is used to displace or drive oil from the oil bearing sand or rock toward and to the producing wells. The steam, which may initially be superheated, is injected into the oil-permeated layer to cause a re-pressurization of the reservoir. As the steam moves away from its initial injection point, its temperature drops and the quality of the steam decreases with the steam eventually condensing into a hot water layer. Additionally, some of the lighter hydrocarbons may be distilled out of the crude oil as it undergoes displacement at the interface between the steam/hot water and the crude oil. The steam injection can be continuous or on an intermittent start-and-stop basis.
In addition to the use of water and steam to effect reservoir re-pressurization and the driveout of the crude oil toward the production wells, carbon dioxide and nitrogen have also been used for the same purpose.
One problem associated with water, steam, or gas driveout techniques is the identification of the boundary or interface between the driveout fluid and the crude oil. In an optimum situation, the boundary between the driveout fluid and the to-be-displaced crude oil would move in a predictable manner through the reservoir from the injection points to the production wells to maximize the production of crude oil. The geology of a reservoir is generally complex and non-homogeneous and often contains regions or zones of relatively higher permeability sand or rock; these higher permeability zones can function as low-impedance pathways for the pressurized driveout fluid. The pressurized driveout fluid sometimes forms low-impedance channels, known as xe2x80x98theftxe2x80x99 zones, through which the pressurized fluid xe2x80x9cpunches throughxe2x80x9d to a producing well to thereby greatly decrease the recovery efficiency.
The ability to identify the position of and the often indistinct interface or boundary between the to-be-displaced crude oil and the pressurized driveout fluid, to track the velocity and morphology of that boundary, and to effect control thereof would substantially enhance secondary oil recovery.
Various techniques have been developed for gaining an understanding of the configuration of the sub-surface geology of an oil-containing reservoir. The dominant technique involves seismic echoing in which a pressure wave is directed downwardly into the sub-surface strata. The initial interrogation wave energy is typically created by the detonation of explosives or by specialized earth-impacting machines. The interrogation wave radiates from its source point with its transmission velocity affected by the elastic modulus and density of the material through which it passes. As with all wave energy, the interrogation wave is subject to reflection, refraction, scattering, absorption, and dampening effects caused by the material through which it passes and from which it is reflected. The reflected wave energy is detected by geophones spaced from the seismic source point and subjected to processing to yield a model of the reservoir. This technique is highly developed and well-suited for detecting sub-surface structures that may be favorable to the accumulation of oil or gas.
Other techniques for investigating sub-surface geology include the use of gravimeters to detect minute changes in the magnitude of the gravity vector for the purpose of detecting sub-surface structures that may be favorable to the accumulation of oil or gas.
The various devices and techniques used to interrogate sub-surface strata have led to significant advances in the ability to create a 3-dimensional model or simulation of the reservoir. However, existing sensing technologies are unable to detect the location and morphology of the boundary or interface between the pressurized driveout fluid and the oil or natural gas in those reservoirs undergoing secondary recovery. Information as to the position, morphology, and velocity of the boundary would be of substantial value in optimizing recovery of the hydrocarbons undergoing recovery, especially in efficient utilization of the driveout fluids.
In view of the above, it is an object of the present invention, among others, to provide a system and process for improving the recovery of fluid hydrocarbons, such as oil and natural gas, from an oil and/or gas reservoir in which the reservoir is undergoing re-pressurization.
It is another object of the present invention to provide a system and process for secondary hydrocarbon recovery in which a pressurized fluid is used to drive oil and/or natural gas from the reservoir to a producing well.
It is still another object of the present invention to provide a system and process for secondary oil recovery in which the boundary or interface between the to-be-recovered oil and a pressurized fluid driving the to-be-recovered oil can be identified.
It is a further object of the present invention to provide a system and process for secondary hydrocarbon recovery in which the boundary or interface between the to-be-recovered hydrocarbon and a pressurized fluid driving the hydrocarbon can be identified and subsequently controlled to maximize recovery.
In view of these objectives, and others, the present invention provides a system and process for secondary oil or gas recovery in which a reservoir is pressurized with a driveout liquid or gas and the boundary or interface between the driveout fluid and the to-be-displaced hydrocarbon material is monitored over time by sensing the changes in density across the boundary with a gravity gradiometer. Sensed changes in the position, extent, velocity, and morphology of the boundary, including the formation of incipient theft zones, allow for control of the injected driveout fluid to optimize recovery efficiency.
A hydrocarbon reservoir undergoing secondary recovery is subject to an initial gravity gradient survey during which a gravity gradiometer takes gradient measurements on the surface above the reservoir to define an initial data set. At some time in the future, a second gravity gradient survey is conducted to provide a second data set. Differences between the first and second data set yield information as to sub-surface density changes associated volt displacement of the gas or oil and the replacement thereof by the driveout fluid. Subsequent gravity gradient surveys similarly displaced in time during the injection of the driveout fluid yield additional information as to the position, morphology, and velocity of the interface allowing an oil field manager to control the number of injection sites and the temperature, pressure, and volume of injected fluid to thus optimize recovery efficiency. The manager can also determine the desirability of drilling new wells, their locations, their segmenting, and desirable orientations of each segment.
In the preferred implementation of the invention, a plurality of gravity gradient measurement stations are established on the surface above an oil or gas reservoir undergoing secondary recovery. A gravity gradient measuring instrument, for example, of the rotating accelerometer type, is positioned at each station in seriatim and data indicative of the gravity gradient at each station is taken to provide a first data set. This first data set yields data constituting baseline information as to the gravity gradient over the reservoir as affected by surface and sub-surface density variations, including the gravity-affecting density contrast at the interface between the driveout fluid and the oil or gas undergoing displacement during the time that the measurements are being taken. At some time subsequent to the measurements that yielded the first data set, i.e., a period of time measured in months or years, the measurements are repeated to yield a second data set. Common data between the first and second data sets are indicative of fixed, substantially invariant data representative of the effect on the gravity gradient of the surface and sub-surface geology while non-common data between the first and second data sets is indicative of a time-dependent change in the gravity gradient consequent to movement over time of the interface between the driveout fluid and the displaced oil or gas and possible geologic noise effects.
After mitigating geologic noise effects, information as to the movement of the interface or boundary is used by an oil field manager to control the number of injection points including volume, pressures, and temperatures to control and improve hydrocarbon recovery.
A particular advantage of the present invention is that the necessity of dealing with invariant common data is substantially eliminated. Only the differences between subsequent sets of data, i.e., the time-lapse gradient data, need to be interpreted in terms of the position of the interface between the driveout fluid and the hydrocarbons undergoing displacement, or, more generally, in terms of the change in the saturation of the various materials in the pore spaces of the reservoir rocks.
An additional advantage of the present invention is that the inherent ambiguity of obtaining sub-surface density information from gradient data is reduced because of the knowledge that the density changes are taking place in only those parts of the sub-surface in which driveout fluids are being injected.
The present invention advantageously provides a system and process for secondary oil recovery that allows observation via measurement of gravity gradients associated with the boundary between the driveout fluid and the to-be-recovered hydrocarbon material in such a manner that recovery efficiency can be optimized.
Other objectives and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings, in which like parts are designated by like reference characters.